Horizontal fragmentation

Assuming that there are only two property types, Flat and House, the horizontal fragmentation of PropertyForRent by property type can be obtained as follows.

• Completeness. Each tuple in the relation appears in either fragment P1 or P2.

• Reconstruction. The PropertyForRent relation can be reconstructed from the fragments using the Union operation:

P1 U P2 = PropertyForRent

• Disjointness. The fragments are disjoint; there can be no property type that is both ‘House’ and ‘Flat’

Vertical fragmentation

• The DreamHome payroll application requires the staffNo and the position, gender, DoB, and salary attributes of each member of staff;

• HR department requires the staffNo, fName, lName, and branchNo attributes.

• The vertical fragmentation of Staff for this example can be obtained as follows:

• Both fragments contain the primary key, staffNo, to enable the original relation to be reconstructed

• The advantage of vertical fragmentation is that the fragments can be stored at the sites that need them

• In addition, performance is improved as the fragment is smaller than the original base relation

Mixed fragmentation

• For some applications, horizontal or vertical fragmentation of a database schema by itself is insufficient to adequately distribute the data.

• Instead, mixed or hybrid fragmentation is required.

• Consists of a horizontal fragment that is subsequently vertically fragmented, or a vertical fragment that is then horizontally fragmented.